In the art of blow molding plastic articles, particularly plastic containers, the initial shape or "parison" which is subsequently blown is generally formed either by injection molding or by free extrusion. Difficulties have been encountered in forming parisons by either one of these two generally accepted methods.
The formation of a parison by free extrusion is limited by the necessity of forming a tubular shape, and it is impossible to initially form a finish, neck or the like on this tube. Further, freely extruded parisons thermally sag as they dangle from the extrusion orifice, a thermal gradient across the axial length of the parison is inherent due to the exposure of the first-extruded portions to the ambient atmosphere while the later-extruded portions are being formed, and abrupt transitions in wall thickness, etc. are difficult to make during continuous extrusion operations.
Injection molded parisons, on the other hand, also have some drawbacks. Typically, a core pin is cantilevered into a surrounding parison mold and the sides and free end of this core pin are encases in plastic to form a blowable shape having a closed bottom. It is difficult to maintain absolute concentricity of the blow pin in the parison mold, resulting in a shape which is not uniformly inflatable during subsequent blowing, snce the cantilevered core pin cannot be supported against radial deflection while still forming the blowable shape having a closed end.
Each system of parison formation has its own advocates, but if it were possible to support the free end of a cantilevered core pin during the injection molding of a parison, particularly for large one-half gallon or gallon containers or the like, the overall advantages of the injection molding system would probably lead to its almost universal adoption.